This invention relates to photographic cameras, and more particularly to a film transport and shutter tensioning mechanism adapted for use in a roll film camera of the type in which the film is contained either in a magazine of normal film frame capacity loaded within the camera body, or in a magazine of large film frame capacity detachable from the rear of the camera body.
Most of the commercially available 35mm film magazines of normal film frame capacity contain 20 to 36 frames of film. In application to photography with a motor driven camera for purposes of taking a large number of picture frames in sequence, however, this normal capacity film magazine is very inconvenient because of the necessity of successive frequent interchanges of the exposed magazine by a new one. Recently, there has been developed a large capacity film magazine containing about 250 frames of film. With the camera associated with such a large capacity film magazine, while the film is transported between a pair of spools contained in the magazine, each frame is advanced by the film transport mechanism incorporated in the camera. As customary, the film transport mechanism is operatively connected to the shutter tensioning mechanism so that a predetermined motion of a single operating member will serve both tension the shutter and to advance the film through the space of one frame. This arrangement creates various problems in operating with the large capacity film magazine while the single operating member is being driven by the motor associated with the camera. For example, when intermittently advancing the film, the deviation of movement of the take-up spool in the magazine from the predetermined relation to the movement of the sprocket in the camera sometimes results in formation of a loop of loose film therebetween. This loop of film may cause the film surface to be scratched by the camera side take-up spool, or to be caught in a space between the camera side take-up spool and the sprocket, thereby making it difficult to perform a smooth film transport operation. This problem becomes more serious in a camera employing a sprocket and spool arranged rotatable in opposite directions relative to each other.
As the complexity of the camera structure increases, the sprocket wheel having eight teeth for engagement with the perforation holes of the film is replaced by a sprocket having six teeth to reduce the dimension of the space which is occupied by the sprocket means. In this case, the sprocket must be turned through four-thirds revolutions to advance the film one frame, and therefore must be provided with an excessively complex and expensive braking device. In order to avoid this difficulty, the driving means for the sprocket is operatively connected to the single operating member, so that a predetermined angular movement (for example, 120.degree. )of the cocking lever constituting part of the single operating member causes a turning movement of the sprocket through fourthirds revolutions which in turn causes advancement of film through the space of one frame.
In a camera of such construction, the sprocket and the take-up spool are arranged to be rotatable in opposite directions to each other, or otherwise the accuracy and reliability in controlling the intermittent advancement of the film would be reduced because of the decrease in the number of sprocket teeth. For this reason, it is particularly important to cope with the problem of preventing the formation of a loop of loose film. According to one solution, a driving torque is imparted by an electric motor to the take-up spool in the large capacity film magazine to exert a tension on the film between the sprocket and the above-identified spool. This tension often causes residual motion of the film in the film gate, resulting in undesirable variations in the quality of image reproduction.